Chats system model - tracker issue

**UX Research insights**
https://github.com/microsoft/vscode/issues/322934 Communicate session and chat model through clear UI structure

**UX/Behavior/UI**

- [Done] - Layout that spatially nests chats inside a session boundary (making the container relationship visible)
- [Catch-all-issue] https://github.com/microsoft/vscode/issues/329321 ---- address the following:
- https://github.com/microsoft/vscode/issues/327579
- https://github.com/microsoft/vscode/issues/327581
- **Bugs:**
- https://github.com/microsoft/vscode/issues/327577 Chats system model - Chat tabs are visually inconsistent with the tabs in the right pane
- https://github.com/microsoft/vscode/issues/327578 Chats system model: New chat button position changes, resulting into annoying behavior
- [Needs design] - Onboarding moment when a user creates their second chat in a session — explaining what just happened

**Validation**
[TODO] Validate new "chats" system model with moderated and unmoderated studies cc: @stevencl